Consolidate documentation on docs.rs

[madsmtm]

There is no clear guidelines in the Rust community on what goes in the README, and what goes on docs.rs. I read Ed Page's blog post on winnow a while back, and the idea of moving everything to docs.rs really resonates with me. I'll copy the core points below.

When exploring documentation options in clap, we found:

• A lot of users want one place to go
• Github is a poor place to store documentation because people naturally look at main and not the version pertinent to them

So a proposal: Let's move the scattered docs we do have from README.md and FEATURES.md to docs.rs.

[madsmtm]

Related proposal, since we're cleaning things up anyhow: Remove the Testers and Contributors table (and disable the wiki), the table is outdated, and I've at least personally never used it myself (though of course it's basically empty for the backends I maintain, so I guess I wouldn't really have a say here).

[daxpedda]

I agree that the Wiki should be removed.